In vacuum and pressure systems, seals are used to maintain the vacuum or pressure. Almost all seals use a gasket. Most of these seals use the force that is used to clamp the sealed parts in contact to also create the seal. This requires many clamps or clamps which apply enough force over a wide area to eliminate gaps in the seal.
Most gaskets are made of an elastic material like rubber, since elastic gaskets can be reused. Ultra-high vacuums require that gaskets have a very low vapor pressure and a very low permeability. Frequently, elastic materials that have very low vapor pressures have relatively high permeabilities, and elastic materials with very low permeabilities have relatively high vapor pressures. In addition, vapor pressure increases as temperature increases so that all elastic gaskets must be kept below 250.degree. F. to be useful in ultra-high vacuum systems. Many vacuum systems are subject to conditions such as extremely hot or cold or radiation environments, which cause elastic gaskets to breakdown. For these reasons, often elastic gaskets are unsuitable for ultra-high vacuums.
Many ultra-high vacuum systems use less-elastic metallic gaskets, since they have a lower vapor pressure than elastic gaskets. Since metallic gaskets have lower vapor pressures than elastic gaskets, they can be used in ultra-high vacuum systems at higher temperatures allowing the system to be degassed by baking. Because metallic gaskets are less elastic, in general they cannot be reused and are subject to leaks caused by stress relief and creep. Some vacuum systems are baked at temperatures up to 400.degree. C. to outgas the system. At this temperature many metals creep. When the system cools down leaks are created by the creep and stress relief.
A few special seals are designed to prevent leaks due to creep and stress relief. A Batzer flange, known in the prior art, provides sufficient elastic deflection and seal force to maintain a seal when creep and relief stress are present. Batzer flanges have a slightly conical flange. The slight deflection in the flange making a conical shape, provides the elastic deflection. One problem with the Batzer seal is that as the size of the seal increases the flange size must increase. Large seals require prohibitively large flanges. Also the seal is along a thin edge which, makes the seal more vulnerable to corrosion or damage.
Another joint design is the "ConFlat." A soft metal gasket is captured in a rigid structure which plastically deforms the gasket. Long bakeout times at high temperatures relieve internal stresses and the force on the seal, allowing the joint to be leak-tight as long as differential thermal expansion is limited. The main problems with these seals is that they require the replacement of the gaskets when the seal is opened, they require many clamps to hold the pieces together and maintain the seal, and the assembly of the parts requires great care and causes much difficulty.
In many applications where seals are remotely created, such as in fission and fusion reactors, the difficulty in creating a seal is of special importance.
Further discussion of seals used in the prior art and their limitations are discussed in Methods of Experimental Physics:, L. Marton and C. Marton, Volume 14 "Vacuum Physics and Technology," Academic Press, 1979 incorporated by reference.